It is universally accepted that, for any new internal combustion engine design, the use of an inwardly-opening (“IO”) poppet for the engine gas-exchange valves is the only sensible architecture to consider. Inwardly opening in this context is taken from the perspective of the engine, and more specifically from the combustion chamber; that is to say, the valves move into the combustion space as they open, rather than away from it. The reasons for this choice are well known, and include the fact that the cylinder pressure acts to improve the valve seat sealing force in a self-assisting manner so that the higher the pressure to be contained, the better the valve is able to seal. Thus, this type of valve has been the standard for well over 100 years and its associated technology has evolved to a high standard.
The following is a list of some of the advantages enjoyed by the inwardly-opening valve (IOV):                Valve seat sealing force increases in a self-energizing manner with cylinder pressure.        Design, manufacture, and application are well understood within the industry.        The valves and their associated valve train components are readily available commodity items.        It works demonstrably well, and meets its objectives—so well in fact that there is no overwhelming incentive to seek improvement.        
This situation notwithstanding, it must be recognized that the inwardly-opening (IO) poppet valve does have some drawbacks, and over the years, alternate designs such as sleeve, rotary, swing, and piston valves have attempted to address these. On every occasion, however, technological improvement to the poppet valve has eventually been able to fight off the challenger, and in so doing, maintain its premier position.
The IOV, however, has some distinct disadvantages, including the following:                The valve head and stem in the port are a restriction to free gas flow. This may be illustrated by noting that steady state flow testing of cylinder heads with the valve in place (at full lift) generally give lower values than those obtained under identical conditions but with the valve removed. Note that the difference between the theoretical flow through a perfect orifice and the actual flow is given as the Coefficient of Discharge (Cd), e.g. a typical Cd=0.7 indicates that the actual flow is 70% of the theoretical flow.        A corollary of the above fact is that for a given gas flow, the valve must be bigger (and therefore heavier) than the ideal. Conversely, an ideal valve, were it possible, would be smaller (taking up less room in the cylinder head) than today's poppet valve.        Since engine power closely correlates with breathing capacity, this encourages the use of the largest possible valves in a cylinder head which often brings the valve very close to the cylinder wall. This can be counter-productive because the proximity of the cylinder wall disturbs the gas flow, resulting in imperfect distribution and a breathing impediment.        In compression ignition (CI, or Diesel) engines, at top dead center (TDC) the piston crown approaches the cylinder head fire-deck as closely as manufacturing tolerances will allow, typically with less than 2 mm clearance. This fact restricts the amount of gas-exchange valve overlap (the duration in crank angle degrees during which both inlet and exhaust valves are simultaneously open) that is possible in a 4-stroke engine if destructive piston-to-valve collision is to be avoided. In the past, for thermal loading reasons this limitation has been a serious impediment to engine durability and power output and may limit the potential for in-cylinder NOx reduction since scavenge flow assists in cooling the cylinder.        An IOV must be designed with a large factor-of-safety associated with it, since intrinsically it is not “fail-safe”. In the event that the valve or its retainer should fail mechanically, it will fall into the cylinder, causing serious derangement to that cylinder and possibly to the engine as a whole. This fact implies that the valve is likely to be more rugged and therefore heavier than might otherwise be necessary.        Valve heads typically occupy a large percentage of the area of the combustion chamber fire deck. Since the IOV is cooled only indirectly, hot valve heads can precipitate premature detonation (uncontrolled ignition and burning), particularly in gasoline engines.        For those engines that are adapted for compression braking (typically heavy-duty CI), a particularly robust valve train is required to open the IOV against cylinder compression pressure (approx. 50 bar). This fact has limited the adoption of cam-actuated compression retarders in medium-duty engines or below.        
The negative aspects to the IOV notwithstanding, it has nevertheless been seen as a very satisfactory fit-for-purpose technological solution for today's engines. Indeed, the hegemonic position enjoyed by the inward-opening valve for the past century makes it seem impertinent, if not unwise, to suggest that it may not be the right solution for the future too. However, in the effort to meet certain future emission legislation, significant changes are coming to the internal combustion engine. These changes will alter the balance of technological attributes required, and in the process will make the inward-opening valve less suited to its use than has been the case in the past. As technology moves into the controlled auto-ignition regime of homogeneous charge compression ignition (HCCI), simply acting as gatekeepers for the inlet and exhaust strokes is no longer sufficient for gas-exchange valves. These valves also need to control effective compression ratio (CReff), and also the quantity of exhaust residuals remaining in the cylinder from the previous exhaust stroke (which may require multiple open/close valve events per cycle). Additionally, practical limits are being encountered with inward-opening valves as engine-specific power ratings increase.
It is beneficial to examine these situations individually in more detail.
Diesel Engine Power Growth:
The desire to increase engine specific-power density (expressed in terms of kW/liter) is pervasive in the industry. In this regard, the gasoline engine has always been dominant and has been the yardstick against which the diesel engine is compared, particularly in the light duty field. Heavy duty engines are typically sold on a “N-dollars per horsepower” basis, where more power within a given engine size category is also desired. It will be apparent that at any given engine speed, if more power is to be produced at the same or similar thermal efficiency, then more fuel must be burned in unit time. Given a similar combustion process, the distribution of the heat energy will be similar for each case considered, with proportionate increases in heat rejection to the exhaust and the cooling systems. Before that heat reaches the cooling system, the heat will be resident in the components most closely associated with the combustion chamber, that is to say, the piston crown and the cylinder head fire deck, including the valve heads. For the cooling system to remove that heat, there must be adequate cooling flow and straightforward heat transfer paths, particularly around the fuel injector and in the exhaust valve bridge region (the narrow section between the two exhaust valves). In the prior art, the desire for more power with its implications of larger valve diameters for better breathing and higher peak firing pressures has now come into conflict with cylinder head low-cycle fatigue (LCF) strength and thermal loading. As a result, to improve head strength and cooling, but to the detriment of breathing, new engines now being designed are obliged to have smaller valve sizes than were previously specified.
A further problem with IOV is that because early in the induction stroke the intake valve is obliged to lag the descending piston, IOVs create negative work for the piston until the valve flow area catches up with the piston rate of displacement. This undesirable throttling effect is discernable and can result in a fuel consumption penalty as great as 2%.
A solution to this dilemma is desired, although it must be pointed out that a smaller valve diameter is not a limitation of itself, since volumetric efficiency can be restored with an increase in inlet port flow capacity (higher coefficient of discharge, or Cd) where possible, or boost pressure, or both. Note however that boost pressure costs energy, and so a solution that does not require higher pressure is preferred.
Controlled Auto-Ignition, or HCCI:
Current engine designs have evolved over the last century in response to customer requirements, fuel availability, metallurgy, and other factors including emission legislation. For example, important driving factors currently are emissions, fuel consumption, durability, and minimized maintenance requirements. Legislation appears to be converging on a zero level of regulated exhaust emissions, and this situation is proving to be problematic for the conventional diesel engine, particularly with respect to nitrous oxide (NOx) emissions, and to a lesser extent with particulate material (PM) emissions. The conventional approach to this problem is to pursue the same path already taken by the gasoline spark ignition (SI) engine, which is to employ a comprehensive suite of exhaust gas aftertreatment (EGA) devices to the engine. The problem with this solution is that it is cumbersome and expensive, and works to put the CI engine at a greater cost disadvantage vs. the SI engine than it already occupies. Thus, another solution is desired.
An alternative solution that appears to be rapidly becoming the industry preference is to adopt one or more of the many advanced combustion concepts that are currently under development in the industry. Broadly, these concepts may be subdivided into those that retain conventional heterogeneous late-injection diffusion combustion (e.g. EPA “Clean Combustion”), and those that employ one or more early injections to enable a controlled auto-ignition (CAI), also known as HCCI. (See: Mello, J P and Linna, J R, “Homogeneous Charge Compression Ignition”, TIAX LLC, 2003.) Both concepts require high levels of exhaust gas recirculation (EGR) back into the cylinder, but the latter approach is currently limited to about 50% of the brake mean effective pressure (BMEP) of the former since it is obliged to operate in a regime that is lean of the flammable range (>approx. 35:1 air/fuel ratio). HCCI has, however, demonstrated very low engine-out levels of NOx and PM, typically better than the first concept, and thus is an attractive path to pursue, particularly if the limited power potential issue can be overcome.
There are, however, many different “HCCI” strategies at this time and it is not clear which ones are likely to see widespread adoption. Nevertheless, a common feature of the advanced premixed auto-ignition combustion systems is that there is no positive initiation for the combustion event, as there is for the SI engine (the spark), or for the CI engine (the introduction of atomized fuel into hot compressed air). As such, other factors have to be manipulated to control the timing of the detonation which otherwise would occur well before TDC, resulting in undesirable negative work.
Assuming an engine of fairly conventional architecture operating on diesel fuel, the challenge is to postpone the start of combustion until just after TDC. Of the many published strategies to achieve CAI, a high level categorization would be between those that employ early injection(s) to achieve the necessary homogenization for clean combustion, and those that attempt late injection in which all the fuel is delivered during the “delay” period (that very short duration of time between the start of injection and the start of combustion). This latter approach has more in common with current engines, since it requires very high injection pressure in conjunction with a special multi-hole nozzle; however, achieving a homogeneous mixture in the short time available is extremely challenging, requiring a very expensive injection system. At the end of the day, the former approach is likely to win since it should be able to employ a much lower-cost injection system; however, both concepts, and particularly the latter, require start-of-combustion controls.
There are a number of parameters that can be manipulated to postpone combustion (when the engine is warm), or advance it (when the engine is cold), but chief among these are CReff and EGR, as pointed out above. In a warm engine, an increase of cooled EGR in the charge will serve to delay combustion, while in a cold engine the exhaust gas heat will serve to advance combustion. Likewise, a lower numeric compression ratio will delay combustion while a higher ratio will advance it. A means to conveniently effect these changes is therefore required.
Within the engine prior art, it is generally perceived that these changes can be made through the active modulation of valve events, sometimes referred to as variable valve actuation (VVA); however, by far the majority of mechanisms that have been proposed for this purpose are much better suited for SI engines that in general do not have the valve-to-piston interference issues that typical CI engines do. Thus it appears that the current mindset within the industry, and therefore the focus of activity, is to adapt SI VVA systems for the CI engine rather than to approach the problem from first principles.
What is needed in the engine arts is a new valve and valve train mechanism that is better suited to enabling CAI conditions in the diesel engine, and at a cost that will not disadvantage the CI engine vs. its SI counterpart.
Desired Functionality:
The following is a brief review of the ideal functionality that a valve mechanism should possess. Recall that the objective for future engines is to deliver zero exhaust emissions with minimum fuel consumption, without giving up any of the desirable attributes of power and responsiveness that current engines provide. A number of new and little-used older strategies in addition to CAI that are being widely discussed within the industry are expected to be utilized to achieve the objective, and a common theme is that they all require WA. More specifically, the VVA needs to be particularly flexible so that more than just a single strategy can be employed, suggesting that valve “mobility” will be an important attribute in the future. Mobility in this context implies the freedom to open or close any intake or exhaust valve at any time in the cycle without undue difficulty or hindrance. Such freedom is clearly impossible in an IO interference engine.
In the same way that flexibility of injection characteristics provided by common rail fuel injection systems have revolutionized the diesel engine in recent years, so it is thought that flexibility in valve event timing will bring another step change improvement by enabling advanced strategies hitherto thought impossible. Among the strategies being discussed are included:
1. The Atkinson Cycle (Late Exhaust Valve Opening, or EVO, giving high expansion ratio).
2. The Miller Cycle (Early or late Intake Valve Closing, or IVC, to modify CReff in conjunction with external compression).
3. The Air-Hybrid Cycle (Compression regeneration; see U.S. Pat. No. 6,223,846).
4. The Curtil Cycle (Pressure-wave supercharging technique utilizing VVA; see U.S. Pat. No. 5,819,693).
5. Two-stroke, four-stroke, six-stroke, eight-stroke switching.
6. Engine braking (Compression retardation).
Strategies 1, 2, and 3 are primarily aimed at fuel efficiency improvement; strategies 4 and 5 offer performance enhancement particularly in CAI mode; and strategy 6 extends the benefits of compression retardation to engines below the circa 2.0 liter/cylinder, heavy duty category that utilizes it today. An engine of conventional architecture but with a flexible VVA system would be able to adopt the Atkinson, Miller, and Curtil cycles under differing operating conditions, whereas air-hybrid and multiple stroke-switching engines would require additional complexity to function effectively. Note, however, that CAI/HCCI is possible today over a limited operating range with today's engines, but practical implementation is essentially technology-limited; the better and more flexible the technology, the more capable the engine will be.
These requirements suggest that in future CI engine design, thee will be a migration to camless valve trains that offer valve mobility with good refinement; minimal noise, vibration, and harshness (NVH); high reliability; and durability that is at least up to current standards, assuming it is not accompanied by excessive on-cost.
It is a principal object of the present invention to provide a gas-exchange valve system wherein the entire valve port is open to passage of gas therethrough.
It is a further object of the present invention to provide a way wherein camless engines may be confidently enabled.